Deposited films are widely used in the fabrication of modern semiconductor devices. These films provide, for example, conducting regions within a device, electrical insulation between metals, and protection from the environment. The deposited films must meet many strict requirements. The thickness of a deposited film must be uniform over each device and over the large number of semiconductor wafers processed at one time in a reactor. The structure and composition of the film must be carefully controlled and must be reproducible. Finally, the film deposition method must be safe, both to the environment and to the production equipment, reproducible, easily automated, and inexpensive.
Many methods are available for the deposition of films on semiconductor devices. Several common film deposition methods are atmospheric-pressure chemical vapor deposition (CVD), low-pressure chemical vapor deposition (LPCVD), and plasma-enhanced chemical vapor deposition (PCVD). Generally, these film deposition methods use temperatures in the range of 100 to 1000.degree. C. and pressures from atmospheric down to 50 mTorr.
Unfortunately, many of the gases used to deposit films on semiconductor devices, and the gas byproducts produced during deposition, are hazardous (e.g., corrosive, flammable, poisonous, or explosive). This is especially true for low-pressure depositions because the processes typically use concentrated gases.
Often, deposition processes which use pumps to expel effluent from the deposition reactor have safety and maintenance problems associated with them, because the gases in the effluent can dissolve or react in the pumping system, or may react with air, to form particulates within the effluent. The reaction particulates commonly collect in the pumps, thereby drastically reducing the life of the pumping system and associated exhaust system.
Various trap systems have been used to remove reaction particulates from the effluent of a deposition system. One type of prior art trap system uses cooling coils in combination with a filtering arrangement to remove the reaction particulates from the effluent. The cooling coils reduce the temperature of the effluent prior to filtering to enhance the trapping of the reaction particulates by the filtering arrangement.
The effluent enters the trap system through a gas inlet and exits the trap system through a gas outlet. The gas inlet is positioned at a right angle relative to the gas outlet to allow gas expansion within the housing of the trap system. In operation, the effluent passes into a side of the trap system through the gas inlet, flows downward to the bottom of the trap housing to the cooling coils, passes upward through the filtering arrangement, and passes out of the top of the trap system through the gas outlet.
Although the above-described prior art trap system provides for the removal of reaction particulates from the effluent of a deposition system, it suffers from limited gas flow conductance and reduced filtering efficiency. This occurs because one-half of the original pressure of the effluent is lost as the effluent flows through the right angle turn formed by the relative orientation of the gas inlet and the gas outlet on the housing of the trap system.
As a result, this trap system is not suitable for use with low pressure applications such as a low pressure chemical vapor deposition system. The additional pressure reduction of the already low pressure effluent as it passes through the trap system from the gas inlet toward the gas outlet greatly reduces the effectiveness of the filtering arrangement.